<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style>
			body{
				/* 弹性布局 */
				display: flex;
				/* 主轴居中 */
				justify-content: center;
				/* 侧轴居中 */
				align-items: center;
				/* 1vh（view height） = 屏幕的1%高度 */
				height: 100vh;
				/* body “那一圈” 边距值，默认是8px */
				margin: 0;
			}
			.box{
				width: 150px;
				height: 150px;
				background: #ad7;
				display: flex;
				justify-content: center;
				align-items: center;
			}
			.a{
				width: 75px;
				height: 75px;
				background: #fc3;
			}
		</style>
	</head>
	<body>
		<div class="box">
			<div class="a"></div>
		</div>
	</body>
</html>
